Many mechanical and electrical systems have a natural resonance frequency, which can be observed by monitoring the rate at which energy supplied thereto is alternately converted from one form to another. For example, in response to an impulsive force, a tuning fork converts potential energy to kinetic energy at its natural resonance frequency (sometimes referred to herein as its "natural" frequency) thereby emitting acoustic radiation whose fundamental frequency component is the "natural" frequency.
Real systems of this type can be modeled as a natural resonant circuit, subject to losses which remove energy from the natural resonant circuit. These losses can be in the form of heat (as in the case of high hysteresis materials) or radiation (as in the case of acoustic radiation from a vibrating system immersed an air or water). If enough energy is dissipated from the system, no resonance will occur (as when the system is critically damped).
A "self resonant" system includes a feedback mechanism which reintroduces power (in proper phase and frequency) to maintain the system in resonance at its natural frequency, while losses remove power from the system.
It is known to monitor the output of some types of self resonant systems and to use such a monitored signal as feedback to control generation of a driving signal for maintaining the system in resonance at its natural frequency. For example, U.S. Pat. No. 4,275,388 (issued Jun. 23, 1981) discloses a piezoelectric alarm system in which output power is periodically measured (by periodically measuring displacement of the piezoelectric transducer in response to a swept driving signal). The measured output signals are employed to update the frequency of a driving signal, in an effort to maintain the driven transducer in resonance.
It is also known to drive a transducer with sharp-edged voltage pulses, by predeflecting the transducer with an initial one of the pulses, and then applying additional pulses at the rate of one pulse for each assumed (not measured) natural resonance period of the transducer. For example, U.S. Pat. No. 4,376,255 discloses a piezoelectric ultrasonic transducer which is driven by an initial pulse followed by a sequence of pulses. The pulses in the sequence are applied at the rate of one for each assumed natural resonance period of the transducer, but the system does not include a feedback means for measuring the transducer's response to the driving pulses and processing the measured signal to determine the transducer's actual natural resonance frequency.
However, until the present invention it was not known how efficiently to monitor the motion of a driven electro-mechanical acoustic transducer and use the monitored motion signal as feedback to control the driving means to keep the transducer vibrating at its natural resonance frequency. Nor was it known to generate a warning signal when such monitored motion signal indicates that the transducer is not vibrating at a frequency within a selected frequency range. Nor was it known to monitor the peak velocity of such a driven electro-mechanical acoustic transducer, to process the monitored velocity signal to generate a feedback signal indicative of actual radiated energy, and to generate from the feedback signal one or both of a control signal (for driving the transducer in a desired manner) and a warning signal indicating that the transducer has not radiated a selected minimum amount of energy.